262 TROPICAL NATURE, AND OTHER ESSAYS. 



study of the influence of local causes in producing such 

 startling results. 



One somewhat similar case does indeed occur among 

 the Mammalia, two singular African animals, the Aard- 

 wolf (Proteles) and the hysena-dog (Lycaon), both strik- 

 ingly resembling hysenas in their general form as well 

 as in their spotted markings. Belonging as they all 

 do to the Carnivora, though to three distinct families, 

 it seems quite an analogous case to those we have 

 imagined ; but as the Aard-wolf and the hysena-dog 

 are both weak animals compared with the hyaena, the 

 resemblance may be useful, and in that case would come 

 under the head of mimicry. This seems the more pro- 

 bable because, as a rule, the colours of the Mammalia 

 are protective, and are too little varied to allow of the 

 influence of local causes producing any well-marked 

 effects. 



When we come to birds, however, the case is differ- 

 ent ; for although they do not exhibit such distinct 

 marks of the influence of locality as do butterflies — 

 probably because the causes which determine colour are 

 in their case more complex — yet there are distinct indi- 

 cations of some effect of the kind, and we must devote 

 some little time to their consideration. 



One of the most curious cases is that of the parrots 

 of the West-Indian Islands and Central America, several 

 of which have white heads or foreheads, occurring in 

 two distinct genera,^ while none of the more numerous 

 parrots of South America are so coloured. In the small 

 island of Dominica we have a very large and richly- 



1 Pionus albifrons and Chry^otis senilis (C. America), Chrysotis sallcet 

 (Hajti), 



